This invention relates to a motor control system and, more particularly, to a system for controlling the speed of a variable speed motor utilizing multiple control criteria.
There are many motor control circuits available today for controlling the speed of variable speed motors. These systems have used triacs, silicon controlled rectifiers (SRC), and similar switches, whose firing angle is controlled relative to the zero-crossover point of the alternating current waveform, to vary the power supplied to the motor and hence the motor speed.
While such control circuits are rather conventional, there arises a particular problem in some motor drive systems such as those used in centrifuges. Not only must the centrifuges' speed be controlled rather accurately so that the particle sedimentation can be controlled, but also the current supplied to the motor normally must be limited according to several different, often unrelated criteria. Priorities must be assigned so that the criteria having the largest deviation from its set point exercises control over the firing point of the SCR. For example, if the motor current exceeds a certain limit, the duty cycle of the power supplied to the motor should be reduced to prevent overloading the circuits. Often the acceleration and/or deceleration of the centrifuge rotor must be accurately controlled. Finally, in the event of a malfunction, the power supply to the motor must be terminated immediately in a fail-safe fashion.
For the most part, motor control systems of the prior art do not fulfill all of these needs without resort to complex, expensive circuitry. Accordingly, it is an object of this invention to provide an improved system capable of controlling electrical motor speed in accordance with several input criteria.
Another object of this invention is to provide an improved system for controlling a motor speed which system has a degree of noise immunity.